GaN-based FETs using a wide bandgap semiconductor such as GaN, AlGaN, InGaN, AlGaN, AlInGaN and the like have received much attention as a power device for high power application since they are one order of magnitude or more smaller in on-resistance than FETs using Si or GaAs and are hence operable at higher temperature with higher current and can withstand high voltage applications.
One example of a conventional GaN-based FET is shown in FIG. 1. As shown, a heterojunction structure is formed on a semi-insulating substrate 91 such as a sapphire substrate. The heterojunction structure includes a buffer layer 92 of GaN, for example, an undoped GaN layer 93, and an undoped AlGaN layer 94, which is generally much thinner than the undoped GaN layer 93. The undoped GaN layer 93 serves as the channel layer. Optionally, two n-AlGaN contact layers 95 are disposed on the undoped AlGaN layer 94. A source electrode S and a drain electrode D are arranged on their respective contact layers 95. A gate electrode G is formed onto the undoped AlGaN layer 94 and is situated between the source electrode S and the drain electrode D. The contact layers 95 may be unnecessary if satisfactory ohmic contact can be established between the source S and drain D electrodes and the underlying semiconductor layer.
The GaN-based FET device is capable of maximizing electron mobility by forming a quantum well at the heterojunction interface between the AlGaN layer, which has a large band gap, and the GaN layer, which has a narrower band gap. As a result, electrons are trapped in the quantum well. The trapped electrons are represented by a two-dimensional electron gas 96 in the undoped GaN layer. The amount of current is controlled by applying voltage to the gate electrode, which is in Schottky contact with the semiconductors so that electrons flow along the channel between the source electrode and the drain electrode.
Even when the gate voltage is zero, electrons will be present in the channel because a piezoelectric field is formed that extends from the substrate toward the device surface. Consequently, the GaN-based FET acts as a depletion-mode (i.e., normally-on) device. For a variety of reasons it would be desirable to provide an enhancement mode (i.e., normally-off) GaN-based FET. For example, when a depletion-mode FET is employed as a switching device for a power source, it is necessary to continuously apply a bias voltage to the gate electrode that is at least equal to the gate threshold value to keep the switch in the off state. Such an arrangement can consume an excessive amount of power. On the other hand, if an enhancement mode FET is employed, the switch can be maintained in the off state even without the application of a voltage, thereby consuming less power. Unfortunately, while attempts have been made to manufacture GaN-based enhanced-mode FETs, they have generally not been satisfactory because of problems such as poor on-state conductances and poor breakdown voltages.